The over-exploitation of rare earth elements has caused serious desertification and environmental pollution, and in China, ecological restoration of mining areas is receiving increasing attention(Wei et al., 2019 ; Wang et al., 2020). In recent years, with the increase of rare earth mining and smelting production, heavy metal pollution of soil in the surrounding areas has become a growing concern and a worldwide problem(Rodriguez et al., 2009; Frossard et al., 2018). So far, mining activities, especially the mining of metal ores, is a major source of soil heavy metal pollution(Huang et al., 2018). Due to different mining activities and habitat specificities, soil properties and heavy metal contents vary considerably over short spatial distances and elevation gradients (Zhao et al., 2019). Heavy metal pollutants caused by mining can be consumed by the human body through the food chain through soil, water, plants, etc., causing great harm to the human health (Liao and Xie 2007; Vinhal-Freitas et al., 2017). Soil contaminated with heavy metals causes changes in soil physicochemical properties and microbial activity, and microbial activity is more sensitive to heavy metals than animal or plant growth in the same soil, soil microbial biomass, soil enzyme activity and metabolic entropy are soil biological parameters(Liao and Xie, 2007), which are responsive to external conditions such as climate, human behavior and heavy metal pollution, and can reflect soil pollution status to some extent(Tang et al., 2019), It can be used as an effective index to evaluate the ecological impact of heavy metal pollution in soil (Ivshina et al., 2014).
Heavy metal contaminants have been shown to be harmful to soil microorganisms, and soil heavy metal contamination may lead to significant changes in microbial diversity, structure and activity(Filip, 2002; Nacke et al., 2014). Microorganisms make an important contribution to the maintenance of terrestrial ecosystems and their biodiversity because the enzymatic activity of soil microorganisms and soil microbial biomass control the cycling and storage of nutrients in the soil(Li et al., 2017). However, the presence of heavy metals in the mining process brings great pressure to microorganisms, which must survive in a heavy metal contaminated environment, thus having a greater impact on microbially mediated soil nutrient cycling(Khan et al., 2010). Both the concentration of heavy metals in the soil and their biological effectiveness influence the toxicity of heavy metals(Kenarova et al., 2014). Soil pH can influence the sorption of metals by substances in the soil, such as organic matter, by altering the surface charge and dissociability of heavy metal sorbents, which in turn affects the bioeffectiveness and toxicity of heavy metals to microorganisms(Bang and Hesterberg, 2004). Heavy metal contamination reduces soil microbial biomass, diversity and biochemical activity due to negative selection of microorganisms sensitive to heavy metal pollutants and inhibition of microbial metabolic activity(Azarbad et al., 2016; He et al., 2016; Yao et al., 2017), hence, low microbial biomass and slower soil organic matter decomposition activity in heavy metal contaminated soils due to low microbial biomass, functional diversity and metabolic efficiency of heavy metal tolerant bacteria in soil microbial communities(Mergeay, 2000). Heavy metals, soil and bacteria interact in a complex manner, and soil microbial communities play an important role in determining soil quality and regulating soil physicochemical properties(Guo et al., 2017); Therefore, soil microbial activity is often considered as a sensitive and effective indicator of mine ecosystems(Liao and Xie, 2007; Liu et al., 2016).
Soil enzyme activity is often considered as a sensitive biological index to evaluate soil quality(Spohn and Kuzyakov, 2014). Studies have shown that redox enzymes and hydrolytic enzymes can be mainly used to evaluate heavy metal pollution. In general, high concentrations of heavy metals could degrade soil cells, destroy soil microbial communities, and inhibit soil enzyme activity(Ciarkowska et al., 2014). Also, catalase is able to break down H2O2 and protect organisms from damage. In addition, catalase has been used as a bioindicator to detect the presence of various heavy metal contaminants(Xian et al., 2015; Hu et al., 2014). Sucrase activity reflects the ability of the soil to break down sucrose and free monosaccharides, which are the main source of energy for soil microorganisms (Frankenberger and Johanson, 1983). Thus, enzyme activity can be used to indicate improvements in the rehabilitation of soils after mining(Schimann et al., 2012). Enzymatic activities are also used for determining the effect of various pollutants including heavy metals on soil microbial quality(Shen et al., 2005; Khan et al., 2007). Studies have shown that heavy metals (Zn, Cu, Ni, V and Cd) in soil would reduce the activities of soil urease, alkaline phosphatase and xylanase (Spohn and Kuzyakov, 2014). It was also found that soil microorganisms polluted by lead-zinc tailing dams reduced urease activity. Enzyme activity varies with the presence of heavy metals, and it depends on different soil properties, heavy metal types and concentrations. Therefore, the integration of multiple enzymes broadly representing microbial metabolism into a comprehensive index is necessary to assess both the toxicity levels of heavy metals in soil microcrops and the ecological impact of heavy metal contamination in soil systems.
Microorganisms in soil contaminated by heavy metals have strong adaptability and viability. The emergence of heavy metal resistance genes in complex microbial communities under heavy metal stress reveals the biological processes and strategies necessary for the survival of microorganisms in extreme environments(Xavier et al., 2019; Thomas et al., 2020). Many bacteria have evolved genetic adaptations to adapt to their environment and acquire metal resistance, multiple genes such as cadB, chrA, pbrA, MerA and NiCoT have reported systerms for bacterial resistance and detoxification, respectively for cadmium, chromium, lead, mercury and nickel, as well as in the involvement of transport of transition metals(Janssen et al., 2010; Das et al., 2016). In response to environmental pollution threatens the survival of microorganisms with various resistance mechanisms(Han et al., 2020), such as metal efflux pump mediated transport, metal produced by permeation barrier, the transformation of heavy metals by intracellular and extracellular enzymes and detoxification, this makes the microbes can by increasing the resistance mechanism of genes to expand their niche in heavy metal contaminated soil(Guo et al., 2018; Xi et al., 2021).
Due to mining activities, there are significant differences between heavy metal and physical and chemical properties in different regions(Kenarova et al., 2014), the microbial community structure will also be adjusted to adapt to different habitats. (Pérez-de-Mora et al., 2006; J Kozdroj, 2001). At present, the effects of tailing waste accumulation on the distribution of heavy metals and bacterial communities remain unclear. As a rare earth ore in the north, this region has a special habitat, it is of great significance to study the effects of rare earth mining on soil physical and chemical properties, heavy metals and soil bacterial communities. The present study aimed to (1) evaluate the effects of manganese (Mn), copper(Cu), lead (Pb) etc. on soil enzyme activities, microbial function, community diversity in rare earth mining areas; (2) assess whether these microbial characteristics can be used as possible indicators of soil pollution by heavy metals and (3) analyze the influence of heavy metal pollution on the abundance of heavy metal resistance genes in soil in different regions.